1. Field of the Invention
This invention relates to the arrangement of sensors, employed with light sources, for detecting whether or not documents are present in the pathway of a document processor. It relates particularly to the arrangement of the light source and the sensor on one side of the pathway opposite an opaque drum having a semi-polished surface, recessed from its outer surface, which provides a good reflection from the light source to energize the sensor when no document is present. The arrangement is such, when a document is present, that the reflection is diverted along a separate path with reduced intensity and the sensor is not energized.
2. Description of the Prior Art
The prior art includes configurations of light sources, such as LED's, which are arranged on one side of a document path so that, when no documents are present, light will be transmitted to phototransistors on the opposite side of the document path. When documents break the beam, logic circuitry associated with the phototransistors provides suitable outputs. An arrangement of this kind employing phototransistors, positioned opposite to LED's, to receive transmitted light is not usable in the environment of interest in the present instance in which an obstruction exists in the form of an alignment drum through which light cannot be transmitted between the LED and the phototransistor.
Other prior art arrangements employ a light source and a photosensor which are positioned on the same side of the document path in such a way that light from the light source is reflected from the surface of a document into the sensor which then produces an output. While such an arrangement may appear to be usable with an opaque drum, as in the environment of interest in the present case, known prior art systems have not proven to be satisfactory, since they necessitate maintaining the sensor pair in a very precise alignment with respect to the reflecting surface for reliable operation and require special drive electronics which is different than that used with the track system of interest.
The present invention avoids the difficulties inherent in the prior art referred to above. It combines the use of common drive electronics with the off-the-shelf light source/sensor combinations, such as LED/phototransistors or LED/photo diodes, used throughout the track system. Each in mounting the light source/sensor is enabled while reliable break-type operation is assured.